Why does the bottom of a lake not freeze in severe winter even when the surface is all frozen?

Q. Why does the bottom of a lake not freeze in severe winter even when the surface is all frozen?

A.            The water has large specific heat

B.            The conductivity of ice is low

C.            The water has large latent heat of fusion

D.            The temperature of the earth at the bottom of the lake is high.

Answer: The conductivity of ice is low